Since the advent of mountain biking, there has been a perceived need for a bicycle gearing system that both is resistant to being derailed by debris that is accidentally kicked up into the gear set by a bicycle rider and yet offers a wide range of gearing. Toward this goal gearing systems have appeared in the literature, in which the gears are encased in a protective housing.
In these new systems a drive wheel may be co-journalled with a driven set of gears and, in turn, drive a cog wheel on the bicycle's rear wheel. Unfortunately, these systems do not have as wide a range of gearing options as some mountain biking enthusiasts might desire. Moreover, the inner workings of some gear box designs are fairly complicated, resulting in a gear box that is heavier and more prone to larger frictional losses and rapid wear than is desirable.